The present invention relates to a device for making a master and more particularly to a master making device including a thin film thermal head for making a master by using a thermosensitive stencil or similar thermosensitive medium.
It has been customary to record an image on a thermosensitive recording sheet, stencil or similar thermosensitive medium or make a master out of such a medium by using a thin film thermal head. A so-called planar thermal head, which is a specific form of the thin film thermal head, has a base formed of aluminum and generally referred to as a heat radiator at its bottom. A thin film substrate is formed on the base and formed of alumina ceramics. A heat insulation layer or glaze layer is formed on the thin film substrate and formed of glass. A resistance layer, which generates heat, is formed on the heat insulation layer and formed of a tantalum (Ta) alloy. A common electrode and discrete electrodes, constituting lead electrodes in combination, are deposited on the resistance layer. Portions of the resistance layer surrounded by the common electrode and discrete electrodes constitute heat generating elements arranged in an array in the main scanning direction of the head.
The above planar thermal head is a typical thin film thermal head and easy to produce and low cost. A master making device using the planar thermal head forms part of a digital stencil printer or digital thermal printer and is well known as a simple printing system. A thermosensitive stencil for use in this type of printing device is implemented as a laminate made up of an extremely thin film formed of polyester or similar thermoplastic resin, a porous base, and an adhesive layer adhering them together. The base is implemented by vynilon fibers, polyethylene terephthalate (PET) fibers or similar synthetic fibers, or Japanese paper fibers, flax fibers or similar natural fibers, or a mixture of Japanese paper fibers and synthetic fibers.
It has recently been proposed to use a 30 xcexcm to 30 xcexcm thick stencil thinner than the conventional stencil (about 40 xcexcm to about 50 xcexcm thick) although not as thin as a stencil substantially consisting of a thermoplastic resin film only (about 1 xcexcm to 8 xcexcm thick), and including a porous base containing a great amount of synthetic fibers. The entire porous base of this kind of stencil may be implemented by PET. However, such a stencil brings about a problem when applied to the master making device of a digital stencil printer, as follows. When a platen roller in rotation conveys the stencil, the thermosensitive film of the stencil melted by heat sticks to the surface of the heating generating elements of the head and cannot be conveyed by the platen roller over an expected master making distance, causing a reduced image to be formed in the stencil. This obstructs the faithful reproduction of an image.
To solve the above sticking problem, the following measures (1) through (4) have been proposed:
(1) to apply a lubricant containing, e.g., silicone (Si) to the surface of the stencil expected to contact the head;
(2) to increase the amount of natural fibers contained in the porous base of the stencil for thereby increasing friction to act between the platen roller and the stencil;
(3) to increase the above friction by increasing pressure to act between the platen roller and the head or by increasing the outside diameter of the platen roller; and
(4) to shift the heat generating elements of the head toward the stencil outlet side in an effective nip width formed between the platen roller and the head.
However, the measure (1) causes the lubricant to adhere to and accumulate on a protection layer covering the heat generating elements. Such lubricant reduces the thermal conductivity of the heat generating elements and thereby degrades image quality. Further, during master making or printing operation, the above lubricant melts due to heat generated by the heat generating elements and is forced out toward the stencil outlet side of the head due to the conveyance of the stencil. Subsequently, the lubricant is cooled off and solidified as it moves away from the heat generating elements. Particularly, when a solid image, for example, is continuously formed in a thermosensitive stencil having relatively low mechanical strength by the head of a digital stencil printer, the above repeatedly occurs. As a result, the solidified lubricant accumulates on a common electrode positioned at the stencil outlet side of the head, raising the stencil above the head. The resulting clearance obstructs the heat transfer from the heat generating elements to the stencil and thereby disturbs the master making operation or the printing operation.
The measure (2) is undesirable because natural fibers are susceptible to environmental conditions including humidity. Therefore, the stencil becomes more susceptible to ambient humidity as the amount of natural fibers contained in the porous support increases, degrading the surface smoothness of the stencil and therefore image quality accordingly. This is apt to lower a so-called perforation probability.
The problem with the measure (3) is that an increase in the pressure of the platen roller directly translates into an increase in the mechanical stress to act on the head. This is apt to reduce the service life of the head by, e. g., causing the protection film of the head to come off. On the other hand, the diameter of the platen roller is, in many cases, determined by the size of the thin film substrate of the head. The platen roller therefore cannot have a diameter greater than the upper limit. Moreover, the current trend is toward a smaller thin film substrate capable of noticeably reducing the cost of the head and therefore toward a smaller platen roller diameter. The platen roller diameter therefore cannot be increased beyond a certain limit.
As for the measure (4), the effective nip width noticeably varies in accordance with the instantaneous platen roller pressure as well as platen roller specification including diameter, rubber thickness and rubber hardness. It is therefore difficult to adjust the position of the heat generating elements of the head, taking account of the variation of the effective nip width. Stated another way, because the effective nip width varies every time the pressure and/or the specification of the platen roller is changed, the heat generating elements must be shifted each time. Furthermore, because the effective nip width finely varies due to the platen roller in rotation or the stencil in movement, it is extremely difficult to so position the heat generating elements as to implement optimal perforations in any possible condition.
To reduce the size of the thin film substrate, it is desirable to cut the head from the protection layer to the substrate at a position as close to the heat generating elements at possible. However, because cutting even the substrate of the head by etching is difficult, a cutting device is required which would lower production efficiency and increase the cost. Moreover, the cutting device leaves noticeable burr on the cut end of the substrate and is therefore required to cut the substrate at a particular position with respect to the heat generating elements. In addition, burr is apt to scratch or otherwise damage the film surface of the thermosensitive medium.
Presumably, the above problems occur more or less with all kinds of stencils of the type including a film.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 8-67061 and 11-77949.
It is therefore an object of the present invention to provide a master making device obviating a troublesome procedure for positioning the heat generating elements of a thermal head in an effective nip width, and reducing the distance over which a thermosensitive medium is conveyed by being nipped between a platen roller and a thermal head after perforation to thereby obviate a reduced image ascribable to sticking.
It is another object of the present invention to provide a master making device allowing the thin film substrate of a thermal head to be cut without any bur while preventing production efficiency from decreasing and cost from increasing, and protecting the film surface of a thermosensitive medium from damage ascribable to burr.
In accordance with the present invention, a device for perforating a thermosensitive medium in accordance with an image signal to thereby make a master includes a thermal head including a plurality of heat generating elements arranged on a thin film substrate in an array in the main scanning direction. A platen roller presses the medium against the thermal head while in rotation for thereby conveying the medium in the subscanning direction perpendicular to the main scanning direction. The heat generating elements selectively generate heat in accordance with the image signal to thereby perforate the medium. The heat generating elements each have an edge thereof, which adjoins the end of the thin film substrate at a medium outlet side in the subscanning direction, located at a distance of 0 mm to 0.5 mm from the end of the substrate.
The thermal head may be formed with a stepped portion at the medium outlet side in the subscanning direction. In such a case, the edges of the heat generating elements adjoining the medium outlet side are located at a distance of 0.018 mm to 0.5 mm from the end of the stepped portion adjoining the above edges.